The invention relates to a method for continuously producing a product crystallizate, also known as a crystallite or crystallization substance, having a substantially constant particle size distribution and to a system for carrying out this method.
A series of crystallizates, such as ammonium sulphate ((NH4)2SO4) which is mainly used as fertilizer in agriculture to provide nitrogen and sulphur must be produced where possible in a medium or large particle size, thus avoiding fine crystallizate having, e.g., a particle size of less than 1 mm. The relative coarseness ensures effective distribution and, when mixed with other fertilizers, suppresses a tendency towards demixing which would be aggravated by fines in a particle mixture.
Coarse-particle crystallizates can be produced from solutions in DTB (Draft Tube Baffled) or Oslo-type crystallizers. A DTB crystallizer uses internal circulation with conveyance of the crystal suspension through a flow guide tube, and an internal clarification zone which is separated by a separating plate from the internally circulated suspension zone. At the top of the calmed clarification zone, in which only a low entrainment rate prevails, a suspension containing only fine crystals is drawn off, since the sinking rate of crystals having a diameter above the separation particle as per design is greater than the entrainment rate in the clarification zone. Large crystals thus remain in the homogeneously mixed suspension zone of the DTB crystallizer.
The same also applies to the crystallization space of an Oslo crystallizer which likewise comprises a clarification zone. In both cases, the overflow of the clarification zone is drawn off by an external circulation pump, is mixed many times with an undersaturated feed solution, but in particular is overheated, e.g., in a heat exchanger. Fine crystals contained in the overflow of the clarification zone are completely dissolved by the admixture of undersaturated solution and primarily by the increase in solubility as a result of the elevation in temperature. The overheated, externally circulated solution is guided at the lower end or below the flow guide tube back into the DTB crystallizer and is conveyed by the internal circulation, which is greater in quantitative terms, through the flow guide tube to the phase boundary surface of the crystallizer where by reason of the ongoing overheating, solvent is evaporated and consequently the solution becomes oversaturated. With a sufficiently large internal circulation, the extent of oversaturation can essentially be maintained within the metastable range, so that no spontaneous seed generation occurs and the oversaturation decreases exclusively in the coarse crystals present. Since the internal circulation pump can be operated at a low screw rotational speed, the conveyance effected serves to conserve particles and promotes the formation of a coarse-particle product crystallizate in a DTB crystallizer. Nevertheless, small crystals produced by the comminution of particles are continuously drawn off via the clarification zone and dissolved.
EP 0632738 B1 discloses a continuous crystallization method, in which a coarse-particle ammonium sulphate crystallizate can be produced from an oversaturated, aqueous ammonium sulphate solution in a DTB crystallizer. In this method, a suspension consisting of oversaturated ammonium sulphate solution and already formed crystallizate is continuously circulated within the DTB crystallizer in an internal circuit. Evaporation of the solvent (water) continuously creates new oversaturation which is then reduced by reason of the crystallization which occurs. The vapour resulting from the evaporation is drawn off at the head of the DTB crystallizer. From a part which is separated from the internal circuit of the suspension by means of flow guide walls in the upper region of the crystallization space of the DTB crystallizer and in which in contrast to the base region of the crystallizer a clarified solution is present having a solids proportion consisting substantially of seed crystals and fine crystals, a partial flow of clarified solution is drawn off and, after dissolution of the solids proportion contained therein, is guided back into the base region of the crystallization space.
In order to dissolve the solids proportion, a heat exchanger which increases the temperature of the clarified solution and thus the power of the solvent to dissolve ammonium sulphate is incorporated into the external circuit. Moreover, the supply line, through which the new concentrated ammonium sulphate solution can be fed into the crystallizer also issues into the external circuit upstream of the heat exchanger. A partial flow of suspension with the proportion of solids contained therein at the desired particle size of the product crystallizate is continuously drawn off from the base region. The product crystallizate is separated from the mother solution in a thickener and by subsequent centrifugation and the mother solution is then guided back into the DTB crystallizer. In order to increase the generation of a sufficiently coarse-particle crystallizate and improve production with regard to the cyclical fluctuations in particle size, in this method in addition to supplying saturated ammonium sulphate solution, a crystallizate suspension consisting of ammonium sulphate is fed at a constant admission rate from an external source into the crystallizer.
EP 0632738 B1 does not contain any references to the manner in which the suspension is produced, i.e. whether it has been produced by the dissolution of comminuted product crystallizate or in a separate crystallizer. It is merely specified that this suspension must satisfy specific conditions:                The temperature of the suspension fed in must not exceed the operating temperature in the crystallizer. Moreover, the suspension must contain 6-24% by volume of crystallizate, wherein at least 35% of the crystals are larger than 1.2 mm, and the infeed of the suspension is to be of such a rate that the weight of the crystals in the suspension fed in is in the range of 4-25% of the weight of the crystals in the suspension having the product crystallizate, which suspension is drawn off from the base region of the crystallizer.        
This targeted infeed of crystal suspension into a crystallizer to influence the particle size is also defined as seeding.
WO 00/56416 discloses a method for controlling the particle size in the case of continuous mass crystallization, which method is also provided for the generation of coarse-particle ammonium sulphate crystallizate in an Oslo crystallizer or DTB crystallizer and in which in a similar manner to the method in accordance with EP 0632738 B1 seeding is effected with a crystal suspension supplied externally. The seeding product is a crystallizate which in its parameters is produced independently of the current crystallization process and which has an average particle diameter of 0.1-1.0 mm. In this method, the temperature of the seeding product during the addition must also not exceed the operating temperature in the crystallizer, but must be up to 40° C., preferably 10-30° C., lower. All other infeeds and recirculations are free of solids. In specific terms, this means that the feedstock for the generation of the ammonium sulphate product crystallizate is supplied as a preheated solids-free ammonium sulphate feed solution, and that the external circuit consisting of drawn-off solution with a fine solids proportion leads out of the crystallizer initially to a heat exchanger which by elevating the temperature of the solution causes the solids proportion to re-dissolve, before the solids-free solution is then guided back into the crystallizer. The heat energy for heating the heat exchanger is provided by means of the vapour which is drawn off from the crystallizer and which initially is brought to a higher temperature level by vapour compression.
From the base region of the crystallizer, a suspension having a solids proportion at the desired particle size is continuously drawn off and separated by centrifugation into product crystallizate and mother solution, wherein the mother solution is conveyed into an intermediate container from where it is fed back into the circulation line of the external circuit of the crystallizer. The seeding product is added preferably in an amount, of which the solids proportion amounts to 5-30% by weight of the solids discharged in each case from the crystallizer. The solids proportion of the seeding product can be generated, e.g., by mechanically comminuting some of the product crystallizate and/or by means of a separate crystallization stage.
Finally, JP 2005-194153 A discloses a system, which is designed as a DTB crystallizer, for producing ammonium sulphate crystallizate, in which an external circuit for clarified solution is provided, which circuit is connected to a clarification zone of the crystallizer and into which a heat exchanger or a supply line for solvents (e.g., water or undersaturated ammonium sulphate solution) is optionally incorporated for the purpose of dissolving the contained solids proportion. Moreover, the clarification zone is connected directly or indirectly via the external circuit to a further extraction line, by means of which a suspension which contains only fine crystallizate as solids can be completely locked out of the process, in order to obtain, e.g., fine-particle ammonium sulphate product. Furthermore, the uppermost part of the clarification zone has a third extraction line connected to it, through which an excess of seed crystals and very fine crystallizate can be drawn off and guided into a collecting container where required. In order to dissolve the solids, solvent is introduced into the collecting container, from where the solution obtained is guided into a neutralization container, in which sulphuric acid and ammonia are added to the solution and the solution is heated by the neutralization reaction associated therewith. The heated solution is then fed into the crystallizer.